1. Field of the Invention
The present invention relates to a method of producing silicon-carbide single crystals in which a hexagonal silicon-carbide single crystal of the 6H modification is grown by a sublimation recrystallization process.
2. Description of the Prior Art
Silicon carbide (SIC) is a semiconductor material having a wide forbidden energy band of 2.2 eV to 3.3 eV. Silicon carbide has excellent characteristics such that it is thermally, chemically and mechanically very stable and is also highly resistant to radiation damage. In contrast, a conventional semiconductor material, such as silicon, involves a problem that semiconductor elements formed from such a material are impracticable for use under severe conditions, such as high temperature, high output drive, and irradiation of radioactive rays, in particular.
Therefore, as a semiconductor element which can be used under such severe conditions as high temperature, high output drive, and irradiation of radioactive rays, a semiconductor element using silicon carbide is currently attracting attention, with much expectation placed thereon for use in a wide range of applications.
Unfortunately, however, there has not been established any such crystal growing technique as would permit stable supply of high-quality silicon carbide single crystals having a large area on an industrial scale. This has prevented practical application of silicon carbide despite the fact that silicon carbide, as a semiconductor material, has many advantages and good possibilities as above stated.
Hitherto, on a laboratory scale, silicon carbide single crystals of such a size as may just enable a semiconductor element to be formed have been produced by growing such a crystal by using a sublimation recrystallization technique.
However, this technique can only produce silicon carbide single crystals of a small area, and this makes it difficult to control the size and configuration of such a crystal to high precision. Further, it is not easy to control the resulting crystal structure of silicon carbide and the concentration of impurity carriers.
It is also known to employ a chemical vapor deposition technique (CVD process) to heteroepitaxially grow a crystal on a heterogeneous substrate of silicon or the like thereby to produce a silicon carbide single crystal of the cubic modification. According to this technique, it is possible to obtain a large-area silicon carbide single crystal. However, the technique has a limitation that the crystal grown involves about 20% of lattice misalignment relative to the substrate. As such, with the technique it is only possible to grow a silicon carbide single crystal including many lattice defects (of up to 10.sup.7 /cm.sup.2), it being not possible to obtain a high quality silicon carbide single crystal.
Therefore, in order to solve the above noted problems, an improved sublimation recrystallization method has been proposed wherein the process of sublimation recrystallization is carried out using a seed crystal (Yu. M. TAIROV and V. F. TSVETKOV "GENERAL PRINCIPLES OF GROWING LARGE-SIZE SINGLE CRYSTALS OF VARIOUS SILICON CARBIDE POLYTYPES", Journal of Crystal Growth, 52 (1981) 146-150).
The use of this process makes it possible to grow a silicon carbide single crystal while controlling the crystal structure and configuration thereof.
FIG. 4 shows generating rates of several crystal structures during a heating process of a silicon carbide crystal under normal pressure, with heating temperature taken as a parameter. Various crystal structures of hexagonal (6H and 4H modification) and rhombohedral (15R modification) crystal structure will be generated in mixture within the temperature range (2000.degree. C. to 2500.degree. C.) normally used in a sublimation recrystallization process (Philips Research Reports, 18 (1965), pp 161).
Usually, in a sublimation recrystallization method using a seed crystal, it is possible to grow a silicon carbide single crystal by controlling the temperature of the seed crystal, the temperature gradient between the seed crystal and the source material, and the pressure. During that process, generation of hexagonal crystal in modification of 6H and 4H (near 2000.degree. C. in FIG. 9) can be controlled by the temperature, temperature gradient and pressure for crystal growth (Japanese Patent Laid-Open Publication No. 2-48495).
However, for the purpose of developing crystal of hexagonal system in modification of 6H without generating rhombohedral crystal in modification of 15R (near 2450.degree. C. in FIG. 9), the process involves a problem that it is difficult to control crystal formation by such growth conditions as growth temperature, temperature gradient and growth pressure.
In particular, where a high-purity source material is used, or during the stage of growth through nitrogen addition for growing an n-type crystal, 6H and 15R crystal structure tend to grow in mixture. As such, it is not possible to selectively grow a 6H silicon carbide single crystal of the n-type alone.